In procedures of manufacturing an NAND nonvolatile memory, when forming a well region in a substrate, impurity implantation is performed to obtain a channel profile. An example of impurities (dopant) to be used is boron (B).
However, even if the impurity implantation is performed so as to obtain an ideal channel profile, the implanted impurities diffuse due to heating processes performed later, so that the channel profile deviates from the ideal profile. For example, if the impurities in the channel surface diffuse deeper due to the heating processes, the channel potential of unselected NAND strings connected to unselected bit lines does not become higher because depletion layers do not extend. As a result, the memory boosting efficiency becomes poorer, and writing errors at a recording time (programming time) increase.